Low yield ratio uncoated 370mpa grade heavy plate resistant to marine atmospheric corrosion and manufacturing method

Abstract

The application provides a low yield ratio and 370MPa-grade marine atmosphere corrosion resistant heavy steel plate without coating and a manufacturing method thereof. The steel plate is composed of the following components in percentage by weight: C: 0.04% to 0.06%, Si: 0.10% to 0.30%, Mn: 0.30% to 0.50%, P: less than or equal to 0.012%, S: less than or equal to 0.005%, Ni: 2.7% to 3.5%, Cu: 0.35% to 0.85%, Mo: 0.05% to 0.15%, Nb: 0.005% to 0.015%, Ti: 0.006% to 0.030%, Al: 0.015% to 0.040%, B: 0.0008 to 0.0016%, and the balance of iron and inevitable impurities. The components satisfy 22 <= 15[Mo]+6[Ni]+5[Cu] <= 26, the Pcm value (%) is less than or equal to 0.19%, the production method of the steel plate comprises smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling and heat treatment. The yield strength of the steel plate is greater than or equal to 370MPa, the tensile strength is greater than or equal to 510MPa, the elongation after fracture is greater than or equal to 20%, the yield ratio is less than or equal to 0.77, the KV2 impact energy at-40 DEG C is greater than or equal to 200J, the corrosion rate of the steel plate in a marine atmosphere environment in a one-year period is 0.024mm / a to 0.030mm / a, and the thickness of the steel plate is 100mm to 130mm.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials, and particularly relates to a low yield ratio and paint-free 370MPa-grade marine atmospheric corrosion resistant super-thick structural steel for bridges and buildings and a manufacturing method thereof. BACKGROUND

[0002] With the development of bridge construction in China, many coastal and cross-sea bridge projects are under construction or planning. Since the bridge site is in a high-chloride corrosion environment, the use of conventional steel materials is limited, so the demand for marine atmospheric corrosion resistant bridge steel is increasing. The content of Cl - in the marine atmosphere is high, and the environmental temperature and humidity will change repeatedly, which will destroy the passive film generated on the surface of the steel and cause serious corrosion of the steel.

[0003] The main load-bearing part of the bridge is mostly a steel structure. In the marine environment, corrosion will cause the thickness of the structure to decrease, and even stress corrosion cracking will occur, which will greatly affect the safe service of the steel structure. In recent years, there has been a growing demand for the implementation of minimal maintenance of bridge steel structures, so paint-free corrosion-resistant steel has attracted much attention. It is very urgent to reduce the corrosion rate through a stable protective rust layer to achieve the demand of not using paint. At present, there are some studies on marine atmospheric corrosion resistant structural steel at home and abroad, and some patents have been found through retrieval, but the content, production method, performance, and product category of the technical solutions described in the patents are obviously different from those of the present application.

[0004] Chinese patent application No. CN202210535812.0 discloses a "corrosion resistant steel suitable for high-humid-thermal marine environment and preparation method thereof", which solves the problems of high cost or limited corrosion resistance, poor long-term corrosion resistance of the corrosion resistant steel for high-humid-thermal marine environment in the prior art. The chemical composition of the corrosion resistant steel is as follows: C≤0.06%, Si 0.20-0.40%, Mn 0.4-0.8%, Ni 1.0-2.0%, Cr 0.4-0.6%, Cu 1.0-2.0%, Sb 0.2-0.6%, and the rest is Fe. The corrosion resistant steel of the invention has low cost and good corrosion resistance in high-humid-thermal marine environment. The limitations of the patent mainly include: the composition contains Sb element, which has a large smelting difficulty coefficient and is not green and environmentally friendly. The addition of Cr element will cause the corrosion resistance of the steel plate to reverse in long-term corrosion in the marine environment, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0005] Chinese patent application number CN201410299647.9 discloses "a high-performance marine climate-resistant steel plate and a manufacturing method thereof", the composition weight percentage of which is: C 0.040-0.080%, Si≤0.30%, Mn 0.40-0.80%, P≤0.015%, S≤0.003%, Cu 0.15-0.45%, Ni 1.25-1.85%, Mo 0.15-0.45%, Ti 0.007-0.013%, Nb 0.015-0.030%, Als 0.035-0.065%, N≤0.0050%, Ca 0.001-0.004%, the rest being Fe and unavoidable inclusions. The invention adopts a low-alloy weathering steel composition system of ultra-low C-low Mn-high Als-low N-(Cu+high Ni) alloying-Nb micro-alloying-ultra-fine Ti treatment, controls the marine atmospheric corrosion resistance parameters η≥1.36, (%Ni)×(%Mo)≥0.27, Ceq≤0.42%, Ca treatment, and the Ca / S ratio is controlled between 1.0-3.0 and Ca×S0.28≤1.0×10-3, adopts TMCP process, and obtains a marine atmospheric steel plate with excellent strength and toughness, strong plasticity matching, low yield ratio, marine atmospheric corrosion resistance, large heat input welding, and fatigue resistance. The limitations of this patent mainly include: only 370MPa grade steel with thickness≤80mm can be produced, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0006] Chinese patent application number CN202011325158.8 discloses "a building structure steel suitable for marine atmospheric environment and a production method thereof", which provides a building structure steel suitable for marine atmospheric environment, belonging to the technical field of steel plate production. The chemical composition and mass fraction of the building structure steel are as follows: carbon: 0.07%-0.17%, silicon: 0.6%-0.8%, manganese: 0.3%-1.0%, phosphorus: 0.08%-0.15%, sulfur: 0.005-0.035%, copper: 0.15%-0.2%, antimony: 0.1%-0.2%, cerium: 0.0025%-0.0045%; and optionally containing: tin: 0.01%-0.02%, vanadium: 0.05-0.1%, or any one or both of the above, the rest being iron and unavoidable impurities. The building structure steel has excellent corrosion resistance, which can effectively improve the service life and safety of steel structure buildings. The limitations of this patent mainly include: the composition contains Sb and Sn elements, which increases the difficulty coefficient of smelting, and is not green and environmentally friendly. The I index is used to evaluate the weather resistance, which may cause large deviation due to the use range of the formula. The steel grade does not achieve paint-free use in terms of corrosion resistance.

[0007] Chinese patent application number CN202211028402.3 discloses a "production method of marine atmospheric corrosion resistant steel plate", the weight percentage composition of the steel is C=0.03%-0.08%, Si=0.15%-0.30%, Mn=0.60%-1.50%, P≤0.025%, S≤0.010%, Al=0.020%-0.050%, Ti=0.007%-0.030%, Nb=0.010%-0.060%, Cu=0.20%-0.50%, Ni=0.50%-3.50%, Cr≤0.50%, Mo≤0.30%, the balance is Fe and unavoidable impurity elements; the key process steps include heating, rolling, cooling, surface treatment and heat treatment. The invention controls the surface iron oxide scale and rust layer of the casting blank and steel plate, so that the steel plate can quickly obtain a protective film of phosphate, nitrate or chromate with strong adhesion and a uniform and stable oxidation layer protective film containing nickel, chromium, copper or calcium, which can prevent liquid rust flow and play a role of "stopping rust with rust" during use and service of the steel plate. The limitations of this patent mainly lie in: the addition of Cr element causes the corrosion resistance of the steel plate to reverse in long-term corrosion in marine environment, and the steel grade does not achieve paint-free use in terms of corrosion resistance.

[0008] Although there are marine atmospheric corrosion resistant steels in China at present, there are still some deficiencies in the research, mainly as follows:

[0009] (1) Using I value to evaluate the weather resistance of low alloy steel with high Ni and Cr content is not completely correct. The calculation formula of I value is: I=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu) 2 . The industry generally recognizes that the I value of steel with weather resistance is ≥6.0, and the larger the I value, the stronger the weather resistance; but the I value formula is based on a large amount of data published by Larrabee and Coburn, and is obtained by regression and correction; therefore, when using this formula, the chemical composition of the steel should meet the chemical composition range during Larrabee-Coburn test: Cu 0.012-0.510%; Ni 0.05-1.10%; Cr 0.10-1.30%; Si 0.10-0.64%; P 0.01-0.12%. Most of the currently disclosed marine atmospheric corrosion resistant structural steels have Ni and Cr elements exceeding the formula usage range, and using the formula beyond the range may lead to incorrect conclusions.

[0010] (2) Using accelerated corrosion tests or electrochemical tests to evaluate the weather resistance of steel materials is insufficient to reflect the actual situation. These two methods can produce a "comparative result" in a very short time, which is feasible in the steel composition screening process; however, they cannot truly reflect the corrosion behavior and characteristics of materials in actual environments. Due to the high concentration of Cl in the marine atmosphere... - Due to its high content and high humidity, the corrosive environment is more severe than that of ordinary atmospheric environment. Therefore, to determine whether a material has long-term corrosion resistance, it is necessary to conduct exposure tests in a real environment to evaluate and predict the corrosion status of the material.

[0011] (3) The problem of "corrosion resistance reversal" of Cr in marine environments needs to be addressed in marine atmospheric corrosion-resistant structural steels. Among seawater corrosion-resistant steels, Cr-containing low-alloy steels are an important series, and the corrosion behavior of Cr steels in seawater is complex. In 1970, Southwcll et al. reported that 3% and 5% Cr steels exhibited "corrosion resistance reversal" after long-term exposure in seawater near Naos Island in the Panama Canal Zone. This means that the corrosion rate of Cr steel in seawater is lower than that of carbon steel in the short term, but higher in the long term. Chinese researchers have conducted extensive studies on the seawater corrosion of Cr steel. Long-term exposure tests have revealed that Cr steel exhibits corrosion resistance reversal in the seawater of Qingdao, Sanya, and Zhanjiang. Therefore, caution is needed when considering whether to use Cr-containing low-alloy steels for weathering steel materials used in cross-sea bridges.

[0012] (4) Regarding corrosion resistance, none of the relevant patented products that have been searched so far have the characteristic of being paint-free.

[0013] (5) The addition of corrosion-resistant elements Sb, Sn and rare earth increases the difficulty and cost of steel smelting due to low yield, and is not conducive to green and environmentally friendly manufacturing. Although the addition of a certain amount of Sb, Sn and rare earth can effectively improve the corrosion performance of the material, there are still some problems. In the aspect of steelmaking, Sb element is only in the form of block added to the molten steel in the ladle during the process of converter tapping, or in the form of block added to the molten steel from the vacuum chamber of RH refining furnace. After the blocky antimony alloy is added to the molten steel, the antimony alloy is heated and melted, and then dissolved into the molten steel. However, due to the long time required for complete melting of the blocky antimony alloy, the solid solution form of antimony in the molten steel is substitutional solid solution. If the melting process of the antimony alloy occurs on the surface of the molten steel, the melted antimony alloy volatilizes into the air due to the low boiling point of antimony and the characteristics of easy oxidation at high temperature. The existing method of adding blocky antimony alloy has low yield, and the yield of Sb is about 20-80%. The Sb element lost in the air is toxic and can seriously harm the health of steelmaking workers. For most steel grades, the effect of rare earth element micro-alloying is unstable, and the flocculation and yield during the molten steel casting process are unstable, which is a difficult problem. Especially for rare earth steel, the casting nozzle is prone to clogging, and the low-multiple inclusion defects of the billet are serious, which further reduces the qualified rate. Therefore, the addition of Sb, Sn and rare earth increases the difficulty of steelmaking process, and even increases the smelting cost.

[0014] Sb can reduce the strength of the steel and increase the brittleness; Sn steel produces segregation and grain boundary segregation behavior during continuous casting billet solidification, which brings harm to the quality and performance of the steel and greatly reduces the mechanical properties of the steel; therefore, in order to compensate for the performance reduction caused by the addition of Sb and Sn, other alloying elements are necessarily added. As a result, the final result is the increase of cost.

[0015] In summary, the existing technology still has deficiencies in the research on marine atmospheric corrosion-resistant structural steel. SUMMARY

[0016] In view of the deficiencies of the prior art, the purpose of the present application is to provide a low yield ratio and free-coating 370MPa grade marine atmospheric corrosion-resistant heavy plate and a manufacturing method thereof. As a kind of bridge, building and other structure steel, the present application does not add Sb, Sn, rare earth and Cr elements, which reduces the smelting difficulty and avoids the reverse effect of Cr on the corrosion resistance of steel in marine environment. By adding a certain amount of Ni, Cu, Mo and other elements, and adopting controlled rolling and controlled cooling plus two-phase zone quenching and tempering heat treatment process, a heavy 370MPa grade weathering steel with excellent strength and toughness matching is obtained. The advantages are that the material has low yield ratio, the maximum thickness specification reaches 130mm, has excellent marine atmospheric corrosion resistance and low temperature impact resistance, and even can be used free of coating under appropriate Cl - deposition.

[0017] The object of the present application is achieved in that:

[0018] A low yield ratio uncoated 370MPa grade marine atmospheric corrosion resistant super thick structural steel has the following chemical composition by weight percentage: C: 0.04%~0.06%, Si: 0.10%~0.30%, Mn: 0.30%~0.50%, P≤0.012%, S≤0.005%, Ni: 2.7%~3.5%, Cu: 0.35%~0.85%, Mo: 0.05%~0.15%, Nb: 0.005%~0.015%, Ti: 0.006%~0.030%, Al: 0.015%~0.040%, B: 0.0008~0.0016%, and 22≤15[Mo]+6[Ni]+5[Cu]≤26 (the numerical values of each element in the formula do not include %, for example 0.05≤[Mo]≤0.15); the balance is iron and inevitable impurities.

[0019] Further, the components satisfy H=15[Mo]+6[Ni]+5[Cu], 22≤H≤26.

[0020] Further, the components satisfy Pcm value (%)≤0.19%, wherein Pcm (%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0021] Further, the steel plate has a structure of ferrite + tempered bainite, wherein the volume percentage content of ferrite structure is 61%~67%.

[0022] Further, the steel plate has a thickness of 100mm~130mm.

[0023] Further, the steel plate has a yield strength≥370MPa, a tensile strength≥510MPa, an elongation after fracture≥20%, a yield ratio≤0.77, a-40℃KV2 impact energy≥200J, and a corrosion rate of the steel plate in a marine atmospheric environment for one year is 0.024mm / a~0.030mm / a.

[0024] The component design reasons of the present application are as follows:

[0025] C: can play a strengthening role through gap solid solution, effectively improve the tensile strength, but the C content should not be too high, because the increase of C is not conducive to toughness, especially has a great influence on low temperature toughness. The present application adds a certain amount of alloying elements, which improves the strength through solid solution strengthening and precipitation strengthening, so it is not necessary to add too much C. At the same time, lower C content can reduce the hardenability of steel during welding, avoid cracking, and thus improve the weldability, so the present application considers that the C content is controlled in 0.04%~0.06% is more appropriate.

[0026] Si: is one of the deoxidizing elements in steel, and Si has a strong solid solution strengthening effect, can purify ferrite, reduce the content of pearlite, and is beneficial to reduce the bainite effect of the base material, but excessive Si will deteriorate the toughness of the steel. In view of the beneficial effect of appropriate Si on the corrosion resistance of the steel, the control of Si content in the present application is 0.10% to 0.30%.

[0027] Mn: through solid solution strengthening to improve the strength of the steel, reduce the austenite transformation temperature, inhibit the phase transformation grain growth before accelerated cooling of the steel plate, play a role in refining the grain, and improve the strength of the steel plate; but too high Mn content will induce segregation, deteriorate the uniformity of the steel plate structure, and the present application considers that the Mn content is controlled in 0.30% to 0.50%.

[0028] P, S: in the present application, they are harmful impurity elements, the lower the better; among them, too high P will lead to organization segregation, and has obvious adverse effect on low temperature toughness, the present application controls P to be ≤0.012%, and the increase of S content will promote the generation and growth of inclusions, and deteriorate the low temperature performance, so S≤0.005%.

[0029] Ni: is a relatively stable element, adding Ni can change the self-corrosion potential of the steel to the positive direction, and increase the stability of the steel. The present application finds through tests that Ni is an effective alloying element for resisting marine atmospheric corrosion, and the effect is significant when the content of Ni is about 1.0% to 3.5%, and the steel can resist various atmospheric corrosion. Under the condition of high salt content environment, after long-term natural exposure, when the content of Ni reaches about 1.5%, the average corrosion depth is greatly reduced. Because the enrichment of Ni in the stable rust layer can effectively inhibit the invasion of Cl - ions, promote the formation of protective rust layer, and reduce the corrosion rate of the steel. In addition, the addition of Ni can avoid the hot working cracking problem of Cu, and also can improve the low temperature toughness of the steel, and the present application considers that the content of Ni is controlled in 2.7% to 3.5%.

[0030] Cu: as the most important alloying element in weathering steel, the purpose is to improve the corrosion resistance of the steel. After adding Cu in the steel, it has superior corrosion resistance to plain carbon steel in rural atmosphere, industrial atmosphere or marine atmosphere. It is worth noting that Cu has obvious effect in offsetting the harmful effect of S in the steel; but the content of Cu should not be too high, because the copper-rich liquid layer will be enriched under the oxide scale of the base iron during the high temperature heating process of copper-containing steel at 1100-1200℃, and this "copper-rich liquid" penetrates along the austenite grain boundary to the interior, which is easy to produce cracks on the surface during rolling process. In view of the effect of alloying element Cu in reducing the corrosion rate of the steel, the present application considers that the content of Cu is controlled in 0.35% to 0.85%.

[0031] Mo: is an alloying element that can effectively improve the atmospheric corrosion resistance, when the steel contains an appropriate amount of Mo, the corrosion rate of the steel in the atmospheric corrosion environment (including industrial, marine and rural atmosphere, especially industrial atmosphere) can be reduced by more than one half. Adding Mo can also effectively improve the pitting corrosion resistance of the steel. In the marine environment, Mo is desorbed from the steel to become negative molybdate ions, which are negatively charged Cl - , and the penetration of Cl - is inhibited from the perspective of electrochemistry. Mo element can effectively eliminate temper brittleness, so that it has good impact toughness, and can improve the hardenability of the steel. Considering the strength, the application considers that the Mo content is controlled in the range of 0.05% to 0.15%.

[0032] Nb: The role in the application includes (1) precipitation strengthening, precipitation before accelerated cooling during rolling, pinning grain boundaries, promoting nucleation, effectively refining grains, thereby improving strength and toughness; (2) reducing the austenite transformation temperature, which can refine the grains; however, too high Nb content will increase the cost, and the application considers that the Nb content is controlled in the range of 0.005% to 0.015%.

[0033] Ti: can play a solid N effect, form a precipitate phase mainly composed of TiN, and can inhibit the grain growth of austenite under high temperature conditions. Because the solid solubility of Ti is low, it is easy to appear in the form of interphase precipitation during the transformation from austenite to ferrite, thereby improving the strength. However, too much Ti will reduce the toughness of the steel, and the application considers that the Ti content is controlled in the range of 0.006% to 0.030%.

[0034] Al: is a strong deoxidizing element, and can also combine with N to form AlN, which can refine the grains, improve the low-temperature impact toughness, and reduce the brittle transition temperature of the steel. When the Al content exceeds 0.040%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, the deoxidization is insufficient, and Ti and other easily oxidized elements will form oxides. The application considers that the Al content is controlled in the range of 0.015% to 0.040%.

[0035] B: can delay the ferrite transformation, and is an effective element for improving the hardenability of the steel, but it is easy to segregate at the grain boundaries. When the B content is too high, it will precipitate at the grain boundaries, reduce the grain boundary strength, and deteriorate the toughness of the steel. The range is controlled in the range of 0.0008 to 0.0016%.

[0036] The 370MPa marine atmospheric corrosion resistant steel described in the application has the following relationship between the content of the weathering elements Mo, Ni and Cu: H = 15[Mo] + 6[Ni] + 5[Cu], and 22≤H≤26.

[0037] To ensure good weldability, ensure that the Pcm value (%) is ≤0.19%, and C≤0.06%, wherein Pcm (%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0038] The second technical solution of the present application is to provide a manufacturing method of a low yield ratio and 370MPa grade marine atmosphere corrosion resistant extra-thick steel without coating, which comprises smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling and heat treatment.

[0039] Smelting

[0040] The RH degassing time control is performed during the refining of the present application, and the RH vacuum circulation time is ≥15min. Through long-time vacuum treatment, the [N] of the molten steel is controlled to be ≤0.0040%, the [O] is controlled to be ≤0.0010%, and the [H] is controlled to be ≤0.00015%. The characteristics of the continuous casting of the present application are as follows: the target superheat of the tundish is controlled to be ≤35℃; the whole process is protected by pouring, and the casting speed of the continuous casting billet is controlled to be 0.6-0.8m / min. The electromagnetic stirring or light pressing down is used during the continuous casting of the billet with a thickness of 360mm, so as to reduce the center segregation. In order to avoid the cracking of the billet, the billet is subjected to stack slow cooling after being discharged or is subjected to slow cooling in a slow cooling pit, so as to effectively remove the hydrogen content and the casting internal stress in the billet.

[0041] Slab heating

[0042] The billet is loaded into the heating furnace at a furnace temperature of 650-750℃, so as to keep the internal and external temperatures of the billet consistent at the low temperature stage, and to prepare for the uniform organization at the high temperature stage. The heating rate of the billet is controlled to be 4-6℃ / min during the subsequent heating process, so as to avoid the uneven heating of the internal part of the billet due to the rapid heating. The heating section temperature of the continuous casting billet is 1270-1310℃, the soaking section temperature is 1220-1250℃, and the soaking section holding time is 4.0-5.6h, so as to make the C and N compounds fully dissolved, especially the solid solution of the Mo element, and to avoid the abnormal growth of the as-cast organization. At the same time, the austenite grains are fully grown, so as to provide sufficient deformation power for the austenite deformation. The billet size should be designed to ensure that the width of the billet after the steel rolling is not more than the length of the roll body of the rolling mill.

[0043] Rolling

[0044] The starting temperature of the rolling in the recrystallization zone is 1150-1180 DEG C, the cumulative reduction is 28%-39%, the final rolling temperature in the recrystallization zone is greater than or equal to 950 DEG C, and the intermediate blank thickness is 2.0-2.2 times of the thickness of the finished product. The rolling temperature and deformation process in the recrystallization zone make the austenite grains recrystallize and inhibit the grain growth, the multi-pass large reduction deformation is used to promote the recrystallization of the austenite and achieve the grain refinement target. The starting temperature of the rolling in the non-recrystallization zone is controlled at 790 DEG C-860 DEG C, the cumulative reduction is 50%-55%, and the final rolling temperature is 785 DEG C-815 DEG C.

[0045] Controlled cooling

[0046] The steel plate is cooled by water cooling after rolling, the starting cooling temperature is 730 DEG C-770 DEG C, the cooling speed is greater than or equal to 7 DEG C / s, the final cooling temperature is less than or equal to 300 DEG C, and then the steel plate is air cooled. Compared with the direct air cooling after rolling, the accelerated cooling can reduce the tendency of the austenite growth, so that the austenite grains are small, and finally the microstructure is small.

[0047] Heat treatment process

[0048] The steel plate after rolling needs to be quenched in the two-phase zone + tempered for the quenching and tempering heat treatment; the quenching heating temperature is 790 DEG C-820 DEG C, the total furnace time is 2.8 min / mm-3.2 min / mm, and then the steel plate is quenched by water cooling; the tempering heating temperature is 450 DEG C-500 DEG C, and the total furnace time is 2.3 min / mm-2.7 min / mm. The final state steel plate of the steel is composed of ferrite + tempered bainite, and the volume percentage content of the ferrite microstructure is 61%-67%.

[0049] The present application has the advantages that:

[0050] The components of the present application improve the toughness of the material by low C and low Mn design, inhibit the austenite grain growth by Ti element, refine the grains by promoting the nucleation in the austenite transformation process, reduce the segregation, and improve the microstructure uniformity; and the corresponding production process solves the problem of high strength, good weather resistance and low temperature toughness.

[0051] 1. The smelting and continuous casting process scheme of the present application realizes low P and low S control, improves the quality of the casting blank, thereby improving the performance of the final product, and a reasonable cooling process system is used after rolling to obtain appropriate (Nb, Ti) (C, N) compound second phase precipitates to make up for the insufficient strength caused by low C and Mn. The thickness of the steel plate realizes the coverage of the special thick 100 mm-130 mm specifications.

[0052] 2.The low yield ratio marine atmospheric corrosion resistant structural steel plate without coating according to the application has a yield strength of greater than or equal to 370 MPa, a tensile strength of greater than or equal to 510 MPa, an elongation after fracture of greater than or equal to 20%, a yield ratio of less than or equal to 0.77, and a KV2 impact energy at -40 DEG C of greater than or equal to 200 J.

[0053] 3.The reasonable proportioning of Cu, Ni and Mo corrosion resistant elements in the application enables the material to have good marine atmospheric corrosion resistance, and the material has a corrosion rate of less than or equal to 0.030 mm / a in a marine atmospheric environment with a Cu deposition amount of 0.61 mdd. -

[0054] The atmospheric corrosion development of the steel follows a power function rule, and the following formula is used to predict the corrosion amount:

[0055] Y=A·X B

[0056] wherein the A value is the corrosion rate of the application in a marine atmospheric environment in a one-year period, and is mainly related to the environment; the B value represents the development trend of the corrosion; after the A value and the B value of the material corrosion life prediction curve are obtained, the corrosion thickness reduction of the material after 50 years is predicted, and the result is that the corrosion depth of the steel of the application is less than 0.12 mm after 50 years. Therefore, according to the provisions in the Japanese "Unpainted Weathering Steel Bridge Construction Guidelines", the corrosion amount of the steel plate thickness is less than 0.4 mm in a 50-year period, and the steel of the application can be used without coating in the environment.

[0057] 4.The material of the application does not add Sn, Sb and rare earth elements, and the total upper limit of the valuable alloy elements of Ni, Cu, Mo and Nb is less than or equal to 4.515%, which reduces the alloy cost and is easy to smelt and produce, especially avoids the toxic pollution caused by the addition of Sb elements, and is beneficial to green and environmentally friendly manufacturing. BRIEF DESCRIPTION OF DRAWINGS

[0058] Figure 1 The figure is the corrosion morphology of the steel plate of Example 3 of the application after outdoor exposure for one year.

[0059] Figure 2 The figure is the metallographic structure of Example 3 of the application. DETAILED DESCRIPTION

[0060] The application will be further described below through examples.

[0061] ​The adding effect of alloy elements in the application is divided into three categories: the first is to improve the strength of the material, such as C, Mn, Nb, Ti, etc. to improve the strength of the steel through solid solution, precipitation, etc. The use of Nb element in the rolling process and before accelerated cooling inhibits grain growth, thereby improving the strength and toughness. The use of Ti element inhibits the growth of austenite grains during the heating process of the billet and promotes nucleation during the austenite transformation to refine the grains. The second is to improve the weather resistance of the material. Through a large number of tests, it is found that the addition of multiple elements can achieve better corrosion resistance than the addition of a single element. Therefore, the ratio of weather resistance elements Mo, Ni and Cu in the composition should satisfy the following relationship: H = 15[Mo] + 6[Ni] + 5[Cu], and 22 ≤ H ≤ 26 to achieve high weather resistance. The third is to use B element to improve the hardenability, so that the 100mm-130mm thick steel plate can be produced.

[0062] According to the component ratio of the technical scheme, the embodiment of the application is smelted, slab continuous casting, slab heating, controlled rolling, controlled cooling and heat treatment.

[0063] Slab heating

[0064] The cast slab is loaded into the heating furnace when the furnace temperature is 650-750℃, the heating rate of the cast slab in the subsequent heating process is controlled at 4-6℃ / min, the heating section temperature of the continuous casting slab is 1270-1310℃, the soaking section temperature is 1220-1250℃, and the soaking section holding time is 4.0-5.6h;

[0065] Rolling

[0066] The opening rolling temperature in the recrystallization zone is 1150-1180℃, the cumulative reduction is 28%-39%, the final rolling temperature in the recrystallization zone is ≥950℃, the intermediate billet thickness is 2.0-2.2 times the thickness of the finished product, the opening rolling temperature in the unrecrystallized zone is controlled at 790-860℃, the cumulative reduction is 50%-55%, and the final rolling temperature is 785-815℃;

[0067] Controlled cooling

[0068] The rolling is cooled by water cooling, the opening cooling temperature is 730-770℃, the cooling speed is ≥7℃ / s, the final cooling temperature is ≤300℃, and then air cooling is performed;

[0069] Heat treatment process

[0070] The steel plate after rolling needs to be quenched in the two-phase zone + tempered for the quenching heat treatment; the quenching heating temperature is 790-820℃, the total furnace time is 2.8-3.2min / mm, and then water cooling is used for quenching; the tempering heating temperature is 450-500℃, and the total furnace time is 2.3-2.7min / mm.

[0071] Further, the smelting process comprises RH refining, and the RH vacuum circulation time is ≥15 min, and through long-time vacuum treatment, the molten steel can be controlled to have [N]≤0.0040%, [O]≤0.0010% and [H]≤0.00015%.

[0072] Further, the intermediate ladle target superheat in the continuous casting process is ≤35℃; full protection pouring is adopted, the secondary cooling adopts a medium cooling intensity mode, and the continuous casting billet casting speed is controlled to be 0.6-0.8 m / min; electromagnetic stirring or light pressing is adopted during continuous casting, and after the continuous casting billet is discharged, the billet is subjected to stack slow cooling or slow cooling in a slow cooling pit.

[0073] The chemical composition of the embodiment of the present application is shown in Table 1; the heating process of the corresponding embodiment is shown in Table 2; the rolling and cooling process of the corresponding embodiment is shown in Table 3; the quenching and tempering heat treatment process and performance of the corresponding embodiment are shown in Table 4 and Table 5; and the corrosion life prediction results of the corresponding embodiment are shown in Table 6.

[0074] Table 1 Chemical composition (wt%) of the embodiment of the present application

[0075]

[0076] Table 1 (continued)

[0077] Examples H Pcm value % 1 22.1 0.168 2 22.0 0.172 3 24.4 0.161 4 22.3 0.190 5 25.8 0.187

[0078] Table 2 Heating process of the embodiment of the present application

[0079]

[0080] Table 3 Rolling and cooling process of the embodiment of the present application

[0081]

[0082]

[0083] Table 4 Heat treatment process of the embodiment of the present application

[0084]

[0085] Table 5 Performance of the embodiment of the present application

[0086]

[0087] The atmospheric corrosion development of steel follows a power function rule, and the following formula is adopted when predicting the corrosion amount:

[0088] Y=A·X B

[0089] Wherein X: exposure period (years), Y: average plate thickness reduction (mm), A, B: coefficient constant varying according to environment, steel grade composition. A value corresponds to the corrosion rate of the first year, mainly related to the environment; B value represents the development trend of corrosion; after obtaining A value and B value of the material corrosion life prediction curve, the corrosion thickness reduction of the material after 50 years is predicted, which is shown in Table 6.

[0090] Table 6 Corrosion life prediction of low yield ratio uncoated 370MPa grade marine atmospheric corrosion resistant structural steel

[0091] Examples Chloride deposition amount mdd A value B value 50 year prediction mm 1 0.61 0.024 0.32 0.084 2 0.61 0.028 0.37 0.119 3 0.61 0.024 0.32 0.093 4 0.61 0.028 0.37 0.101 5 0.61 0.030 0.29 0.098

[0092] In summary, the steel plate of the present application adopts reasonable Ni, Cu and Mo ratio to improve the weather resistance of the steel, and is matched with corresponding smelting, heating, rolling, cooling, quenching and tempering heat treatment and other production processes to obtain the 370MPa grade high weather resistant bridge steel super thick plate for cross-sea bridge, which has excellent weather resistance, low yield ratio and low temperature toughness. The yield strength of the steel plate is greater than or equal to 370MPa, the tensile strength is greater than or equal to 510MPa, the elongation after fracture is greater than or equal to 20%, the yield ratio is less than or equal to 0.77, the KV2 impact energy at-40℃ is greater than or equal to 200J, the corrosion rate of the steel plate in the marine atmospheric environment under one year period is 0.024mm / a-0.030mm / a. The product has green environmental protection characteristics and can be used without coating under the condition that the chloride ion deposition amount is not higher than 0.61mdd. Therefore, the product has high cost performance and market competitiveness, and has high technical trade value.

[0093] In order to describe the present application, the above-mentioned embodiments are appropriately and sufficiently described by examples, and the above embodiments are only used to illustrate the present application, but not limit the present application. Any modification, equivalent replacement, improvement and the like made by those skilled in the art without departing from the spirit and scope of the present application shall be included in the protection scope of the present application, and the patent protection scope of the present application shall be defined by the claims.

Claims

1. A low yield strength ratio, uncoated, 370MPa grade, marine atmospheric corrosion resistant extra-thick steel plate, characterized in that, The composition comprises, by weight percentage: C: 0.04%–0.06%, Si: 0.10%–0.30%, Mn: 0.30%–0.50%, P ≤ 0.012%, S ≤ 0.005%, Ni: 2.7%–3.4%, Cu: 0.35%–0.85%, Mo: 0.05%–0.15%, Nb: 0.005%–0.014%, Ti: 0.006%–0.030%, Al: 0.015%–0.040%, B: 0.0008%–0.0016%, with the balance being iron and unavoidable impurities. The composition satisfies H = 15[Mo] + 6[Ni] + 5[Cu], 22 ≤ H ≤ 26. The manufacturing method of the low yield strength ratio, uncoated 370MPa grade marine atmospheric corrosion resistant extra-thick steel plate includes smelting, slab continuous casting, slab heating, controlled rolling, controlled cooling, and heat treatment. Slab heating The billet is placed into the heating furnace at a furnace temperature of 650-750℃. During the subsequent heating process, the heating rate of the billet is controlled at 4-6℃ / min. The temperature of the continuous casting billet heating section is 1270-1310℃, the temperature of the soaking section is 1220-1250℃, and the soaking section holding time is 4.0-5.6h. Rolling The initial rolling temperature in the recrystallization zone is 1150–1180℃, with a cumulative reduction rate of 28%–39%. The final rolling temperature in the recrystallization zone is ≥950℃, and the intermediate billet thickness is 2.0–2.2 times the finished product thickness. The initial rolling temperature in the non-recrystallization zone is controlled at 790℃–860℃, with a cumulative reduction rate of 50%–55%, and the final rolling temperature is 785℃–815℃. Controlled cooling After rolling, water cooling is used. The initial cooling temperature is 730℃~770℃, the cooling rate is ≥7℃ / s, the final cooling temperature is ≤300℃, and then air cooling is used. Heat treatment process The rolled steel plate needs to undergo two-phase quenching and tempering heat treatment. The quenching temperature is 790℃~820℃, and the total furnace time is 2.8min / mm~3.2min / mm. After that, water cooling is used for quenching. The tempering temperature is 450℃~500℃, and the total furnace time is 2.3min / mm~2.7min / mm. The steel plate has a microstructure of ferrite + tempered bainite, wherein the ferrite microstructure has a volume percentage content of 61% to 67%. The thickness of the steel plate is 100mm to 130mm; The steel plate has a yield strength ≥370MPa, tensile strength ≥510MPa, elongation after fracture ≥20%, yield strength ratio ≤0.77, and KV2 impact energy ≥200J at -40℃. The corrosion rate of the steel plate in a marine atmospheric environment over a one-year period is 0.024mm / a~0.030mm / a.

2. The low yield strength ratio, uncoated, 370MPa grade marine atmospheric corrosion resistant extra-thick steel plate according to claim 1, characterized in that, The component satisfies the condition that the Pcm value (%) is ≤0.19%, where Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

3. The low yield strength ratio, uncoated, 370MPa grade marine atmospheric corrosion resistant extra-thick steel plate according to claim 1, characterized in that, The smelting process includes refining in an RH refining furnace, with an RH vacuum circulation time of ≥15 min. Through long-term vacuum treatment, the molten steel is controlled to have [N] ≤0.0040%, [O] ≤0.0010%, and [H] ≤0.00015%.

4. The low yield strength ratio, uncoated, 370MPa grade marine atmospheric corrosion resistant extra-thick steel plate according to claim 1, characterized in that, The target superheat of the tundish during the continuous casting process is ≤35℃; the entire process is protected during casting, and the casting speed of the continuous casting billet is controlled at 0.6~0.8m / min; electromagnetic stirring or light pressure is used during continuous casting, and the continuous casting billet is stacked and slowly cooled after leaving the line or slowly cooled in a slow cooling pit.

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